Stabilizer control



Aug. 24, 1937. e. L. MATEER ET AL STAB ILIZER CONTROL Filed April 19,1955 HEAT 5xCHA/VGER STAB/L IZER- I TOWER (A (/X/L IA R Y) An: nvusrEAL-BOILER.

'6 srzAM INLET 25 L IQU'ID C4/ STAB/L zslz .Bo'rrons Lnws 721271. oxLuvs Patented Aug. 24, 193? UNITED STATES rarest @EFEQE STABILIZERCONTROL Application April 19,

4 Claims.

This invention relates to a method and means for controlling thecomposition of the bottoms product of a stabilizer tower which is, as isknown, a distillation tower employed to remove undesired light ends froma hydrocarbon mixture. This invention is applicable to the treatment ofany stock, from a crude to any intermediate fraction thereof. In orderto illustrate the essential features of this process, however, it willbe described particularly with reference to that type of stabilizertower known in the art as a depropanizer which is employed forseparating propane and lighter hydrocarbons from a cracked distillate,the bottoms of which will consist of hydrocarbons, the lightest memberof which is a butane.

In the practical operation of a depropanizer, considerable difliculty isencountered in keeping the concentration of propane in the bottoms at aminimum. This difiiculty is largely attributable to variations intemperature and partial pressure in the stabilizer tower. It iscustomary to analyze the bottoms for propane content to determine whatchanges in the conditions in the stabilizer tower may be necessary. Thismethod of control leaves much to be desired because by the time theanalysis of the sample of bottoms is completed and the determination ofthe required changes in the conditions which resulted in that sample aremade, those conditions oftentimes will have changed quite radicallywhereby the determinations made from the analysis of the sample becomeinapplicable.

The main object of this invention is to provide a process of operating astabilizer tower in which the content of undesirable hydrocarbons(propane in a depropanizer) can be set at a minimum and can bemaintained at that minimum by using any change in the content thereof tobring into operation controlling means for the temperature of thestabilizer tower and to provide an apparatus in which the temperature ofthe stabilizer tower can be so controlled.

It is a further object of this invention to provide a process in whichonly a portion of the stabilizer bottoms is employed for the operationof the temperature control means and in which the content of undesirablehydrocarbons in the bottoms is greatly magnified in the portion used fortemperature control. The advantage of this mode of procedure is that themore highly the content of undesirable hydrocarbons in the bottoms ismagnified in the portion of the bottoms used for temperature control,the more sensitive will be the temperature control and the more 1935,Serial No. 17,238

accurate will be the control of the content of undesired hydrocarbons inthe bottoms.

Further objects and advantages of this invention will appear from thefollowing detailed description of the accompanying drawing, the onlyfigure of which is a diagrammatic elevation of one type of apparatus inwhich the improved process can be carried out.

Referring to the drawing in detail, numeral i represents a stabilizertower of conventional design to which the feed stock is introducedthrough feed line 2 after passing through heat exchanger 3 and fromwhich overhead, consisting in a depropanizer mainly of C1, C2, and C3hydrocarbons, is taken off through line 4 and passed to a condenser, andthe bottoms, consisting in a depropanizer mainly of C4, C5, C6 andheavier hydrocarbons together with small amounts or" C3 hydrocarbons, istaken off through line 5. The stabilizer tower is also provided with theconventional steam reboiler by means of which the temperature to whichthe bottom stock is heated is controlled by passing part of the stockout of the tower through a coil 6 which is arranged in heat exchangerelation with steam coil 7 fed by steam line 8.

According to this invention, a portion of the stabilizer bottoms fromthe outlet line 5 is passed through heat exchanger 9 and through lineii! into a pentane stabilizer tower it maintained at a temperaturesuitable for causing C3 and C4 hydrocarbons to pass off as overhead andthe higher hydrocarbons to pass off as bottoms. The overhead is passedthrough line i2, through condenser it maintained at the temperaturenecessary to condense C4 hydrocarbons, and into a chamber M where the C4hydrocarbons are deposited and from which the C3 hydrocarbons pass offas overhead through line iii of fixed capacity.

Line i5 is provided with a diaphragm valve or any other conventionaltype of pressure regulating valve it, the purpose of which is tomaintain the system under any selected pressure. The advantage accruingfrom the use of this pressure regulating valve is that by maintainingchamber itunder pressure the amount of water condensation necessary toeffect a clear cut separation of propane and butane vapors can be easilyregulated. This pressure regulating valve plays no part in our novelmethod of control except to build up a pressure in the system when it isstarted and to maintain that pressure in the customary manner.

Arranged in the line I5 is an orifice plate It which forms part of arate of flow controller II of conventional design which is connected toline I5 by the usual small bore gas tubes I8 on either side of theorifice plate I6.

The rate of flow controller I I is hooked up with a temperaturecontroller I9, also of conventional design, which, in turn, is connectedto the thermometer bulb in stabilizer tower I and, through line 2|, to adiaphragm valve 22 in steam line 8 which latter is provided with aby-pass 23 around said diaphragm valve.

Although the two control devices are of conventional design, they areoperatively connected for the purpose of this invention in a manner nothitherto usual. As in the usual rate of flow controller, each of tubesI8 is connected to one of the legs of a U-tube 24 containing mercury orany other suitable liquid. One leg of the U-tube is enlarged and haspivoted therein .a

20 bell crank 25, one arm of which carries a float 26, the position ofwhich is controlled by the level of liquid in the enlarged leg of theU-tube and the other arm of which presses against a flap valve 2'!which, in turn, is mounted to move relative to a nozzle 23 on a bleederline 29 branched ofi" from a main air line 30.

Ordinarily, air line 30 is connected to a diaphragm valve which controlsthe rate of now in the line to which the tubes I8 are connected, wherebyany change in the rate of flow of fluid through said line is reflectedin the liquid levels in the two legs of the U-tube and in the amount ofair expelled through the bleeder line 29. In the present case, however,the air line 3!) is connected to a bellows 3I in the temperature controldevice. This bellows which is attached to one end of arm 32, which inturn is pivoted at its other end to a fixed point adjusts the positionof said arm 32 to which is connected the nozzle 33 of a bleeder line 34.The bellows 3| adjusts the position of arm 32 by expanding andcontracting with a change of fluid pressure in line 30. The flap valve35, which operates in conjunction with the nozzle 33 to control theamount of air expelled through the bleeder line, and which is pivoted toa fixed point at its upper end is actuated by the movement of theexpansion coil 36 connected to the thermometer 20 and to said flapvalve. The air line 3'! in the temperature controller is connected tothe diaphragm valve 22 in the steam line 8 as before described. Thus,instead of the position of the nozzle 33 of the temperature controllerbeing set by hand in the conventional manner, it is set by the pressureof air in the air line 30 which in turn is regulated by the flow of gasin the line I 5.

In the operation of this system the parts are set so that thestabilizing tower will operate at a selected temperature, at which acertain percentage of propane will remain in the bottoms. Any change inthe flow of propane through line I5 will change the pressure in line 30,which in turn regulates the position at nozzle 33, as described above.An increase in the flow of propane in line I5 will close nozzle 28 andincrease the pressure in line 30, causing nozzle 33 to be moved awayfrom valve 35. An increase in temperature in the stabilizing tower asmeasured at 20 causes element 36 to tend to close valve 35. The pressurein line 2| depends upon the relative positions of nozzle 33 and valve35. An increase in pressure in line 2|, caused by a closing of nozzle33, tends to cause valve 22 to close, thereby reducing the flow ofheating medium to the steam re-boiler. Since the relative positions ofnozzle 33 and flap valve 35 are governed both by the content of propanein the bottoms of the stabilizer and the temperature of the stabilizeras measured at 26, it follows that a change in either of these factorsis reflected in the flow of heating medium to the re-boiler.

The system described above could be so adjusted that any flow of propanewhatsoever in the line I5 would cause an increase in the feed of steamthrough steam line 8 and a consequent increase in the temperature of thestabilizer tower I. It is preferred, however, to set the system so as topermit a minimum flow of propane through the line I5 so that in theevent that the temperature in the stabilizer tower becomes so high as totend to send off C4 hydrocarbons as overhead, the flow of steam in theline 8 will be reduced with a consequent reduction in temperature of thetower I.

For the sake of illustration, let it be assumed that the temperature intower II is so adjusted as to cause 30% of the feed to that tower topass off as overhead. All the propane in the feed will be contained inthe overhead. Consequently, if the bottoms from the stabilizer tower Icontain 1% of propane, the overhead from tower II will contain about 3%of propane. It follows that any increase or decrease in theconcentration of propane in the bottoms from stabilizer tower I isreflected three-fold in the flow of gas through line l5 and in thecontrol devices.

In addition to controlling the propane content of the stabilizer bottomsfrom stabilizer tower I, this method of control also aifords a positivemeans of regulating the pressure on the second stabilizer tower I I. Ifno gas were released from the second stabilizer tower II, i. e. all theoverhead vapor from tower II being condensed in condenser I3, thepressure control of this tower would depend on the temperature of thedistillate in chamber I4, which, in turn, depends on the cooling watertemperature. In order to avoid the pressure fluctuations due to changesin water temperature, it is necessary to regulate the Water going to thecondensers of the second stabilizer I I or install a pressure regulatorin the vapor line I2 from the stabilizer tower II, unless the abovemethod of control is used.

It is to be understood that this invention is not restricted to theprecise arrangement of procedural details described above but can bepracticed by the utilization of any means for measuring and controllingthe undesired hydrocarbons in the bottoms and utilizing thesehydrocarbons to bring into operation means for controlling thetemperature in the stabilizer tower.

The nature and objects of this invention and a specific embodimentthereof, which are to be taken as illustrative rather than limitative,having been described, what is claimed as new and useful and is desiredto be secured by Letters Patent is:

1. A process for controlling the composition of the bottoms of astabilizing tower, which comprises regulating the temperature conditionsof the stabilizing tower to retain in the bottoms a certain percentageof the highest boiling substance desired in the overhead, removing thebottoms from the stabilizing tower, separating said highest boilingsubstance from a selected portion of said bottoms, passing said highestboiling portion through a passage of fixed capacity, causing the rate offlow of said highest boiling portion through said passage to be directlyproportional to the volume of said highest boiling portion in saidbottoms, and controlling the temperature in the stabilizer tower by therate of flow of said highest boiling substance through said passage.

2. A stabilizing plant comprising a stabilizer 5 tower, means forregulating the conditions in said tower so as to retain in the bottoms acertain percentage of the highest boiling substance desired in theoverhead, means for removing the bottoms from said tower, means forseparating said substance from a selected portion of said bottoms, apassage of fixed capacity through which said separated substance mustpass, whereby the rate of flow of said substance through said passagevaries with the volume of said substance and means operated inaccordance with the rate of flow of said substance through said passagefor controlling the temperature of the stabilizer tower.

3. A stabilizing plant comprising a stabilizer tower, means forregulating conditions in said tower so as to retain in the bottoms acertain percentage of the highest boiling substance desired in theoverhead, means for removing the bottoms from said tower, means forremoving said highest boiling substance from a selected portion of saidbottoms, a passage of fixed capacity through which said highest boilingsubstance must pass upon being removed from said bottoms, whereby therate of flow of said substance through said passage, is directlyproportional to the volume of said substance in said bottoms, means forheating the bottoms in the stabilizer tower, means for controlling saidheating means operated by the pressure of a stream of gas and meansoperated in accordance with the rate of flow of said highest boilingsubstance for regulating the pressure of said stream of gas.

4. A stabilizing plant according to claim 2 in.

which the means operated by the rate of flow of the highest boilingsubstance for controlling the temperature of the stabilizer towercomprises a steam reboiler, a feed line for steam to said reboiler, adiaphragm valve in said feed line, means for conducting a stream of airagainst said diaphragm valve for regulating its position, meanscomprising a bleeder line provided with an exhaust nozzle forcontrolling the pressure of said stream of air, means operated by asecond stream of air for regulating the amount of air expelled throughsaid nozzle, means comprising a bleeder line provided with exhaustnozzle for regulating the pressure of said second stream of air andmeans operated in accordance with the rate of flow of said highestboiling substance for controlling the amount of air given off by saidsecond exhaust nozzle.

GEORGE L. MATEER. GEORGE F. EUSTIS.

